1. Field of the Invention
The present invention relates to devices and methods for increasing the permeability of porous and fractured media with acoustic stimulation, and more particularly in once instance to methane and natural gas extraction from coalbed deposits, and acoustic borehole equipment to increases the gas permeability of the media surrounding the borehole inside faces of exhaust wells.
2. Description of the Prior Art
Methane, firedamp, or natural gas, is a normal constituent of every coalbed deposit, and are formed in situ by Nature when the coalbed takes form. Such gases are adsorbed by the coal, e.g., they occupy the particle surface areas over the entire parent coal matrix. The adsorption surface area of coal can be very large, e.g., about one billion square feet per ton of coal. The gas stored in a coalbed can be significantly more than the gas found in a typical and otherwise similarly sized natural gas deposit. Such methane and carbon dioxide do not freely migrate through the coalbed deposits. They have to be induced to release from the coal, e.g., by venting to a lower atmospheric pressure.
In contrast, methane deposited in sand or sandstone is not adsorbed by the sand material itself, and is usually able to flow relatively well through the cavities, cracks, fissures, and spaces between the sand particles.
Every coal deposit includes some amount of methane that can make mining the coal dangerous. As a general rule, the amount of methane adsorbed is proportional to the grade of the coal. The higher the coal grade, the higher will be the gas content. Also, the deeper the coalbed, the higher will be its gas content. The pressure in the coalbed is proportional to its depth, and the degree of gas sorption increases with such pressure. A desorption isotherm can be used to predict the reduction in pressure, for a given temperature, that will be needed to get the gas to desorb and seep out to exhaust well collectors.
Coal beds are very often inundated with ground water. The hydrostatic pressure of such water will add to the total pressure in a coalbed and a concomitant increase in the gas sorption. The desorption isotherm shows an appropriate level of decrease in the hydrostatic pressure needed to recover the methane from the coal.
In the past, the collection methods and equipment needed to harvest the methane from a coalbed simply did not exist. So no profit could be make from the methane. Such methane had always been considered a nuisance because it poisoned the air the miners needed to breath, and thousands of times it has proved to be explosively deadly. Even today, when modern methods and equipment can be employed to great success, serious and frequent mining explosions and disasters continue to occur that could have been avoided if the firedamp had simply been removed before coal mining operations began. These accidents have been especially common recently in the coalmines of Russia and China. But no coal mine in the world is immune.
Methane production ahead of mining has become a widespread way to protect against methane-related accidents and to increase profits by selling off the collected methane. In fact, harvesting the methane from coalbeds or strata too deep or too poor to support profitable coal production is becoming an attractive way to convert hydrocarbon reserves into revenues.
Coalmine gas production holes were once simply used to help ventilate mines and to minimize the coal-production risk due to mine gases. Now, coalmine operations recognize that profits can be made by gas production and sales. Simply releasing the gas into the atmosphere is a waste of money, and contributes to environmental pollution. In the last time, the experience in mine degasification led to development of projects of gas production independent of coalmine operation.
Widely used methods for coalbed gas production include vertical and horizontal boreholes drilled to degasify the deposits before starting coal mine production, vertical gassers in waste rock, and vertical or directionally drilled boreholes independent of any intent to later mine coal. Prior art methods for coal methane production have included injecting a second gas, such as nitrogen, carbon dioxide, or vitiated air into coalbeds to force out the natural gas. A system of injection and collector holes is drilled to do this.
A number of factors will determine the profitability of gas production from coalbeds. For example, the actual gas content, the pressure in the coalbed, the presence of water, and the “penetrability” all affect how much gas can be recovered and at what cost. A fracturing pattern inside a coalbed, called “cleavage,” is one factor that determines the in-place penetrability. Cleavage and stratification can ease the flow of gases and fluids inside a coalbed.
For example, a coalbed with a low gas content and a high hydrostatic pressure on the desorption isotherm requires extra production of water for every unit of produced methane. Similarly, gas recovery from a coalbed with a very low penetrability requires intense destruction. In many cases, efficient gas recovery is not possible because appropriate production-enhancement technologies do not exist.
The drilling-in of a borehole in a coalbed causes a localized pressure relief and produces a pressure gradient as the methane flows to the output. A diffusion flux is generated through the coal matrix with a laminar flow through fractures the coalbed around the borehole. Ground water is pumped out to reduce the coalbed pressure enough so the gas can desorb from the coal. The faster the water removal, the faster will be the consequential release of the retained gas. The gas volume output that can be realized by an exhaust well is mainly determined by the penetrability or permeability of the wall and bottom faces of the borehole. Such faces behave like a filter matrix, and the important areas involved in restricting the gas flow the most are not more than a few diameters away from the exhaust well in the collector zone. Therefore, the more permeable that such immediate area around the exhaust borehole can be made, the higher will be the volume of gas produced.
Coal has an elastic nature to its solid makeup that can cause the pores in it to close or restrict gas permeation when subjected to large pressure gradients. The pressure gradients are highest immediately around the exhaust well borehole, and the “filter” area at the perimeter radius is minimum. The pressure isobaric curves form concentric cylindrical zones around the core. Those farther from the exhaust well inside faces have the larger surface areas. The pressure gradients are greatest immediate to the exhaust well inside faces, and the surfaces areas are minimum. The combination closes the gas pores and limits permeability nearest the inside faces. Such observation can also be expressed mathematically.
The formula for a pressure gradient distribution in a one-dimensional radial flow from a circular supply circuit with radius Rc, and pressure Pc to a concentric borehole with effective radius rb, and face pressure Pb, is as follows:
            P      ⁡              (        r        )              -          P      c        =                              P          b                -                  P          c                            ln        ⁡                  (                                    R              c                                      r              b                                )                      ⁢                  ln        ⁡                  (                                    R              c                        r                    )                    .      
Such describes a logarithmic pressure distribution between the supply circuit and the borehole at the center. Most of the pressure differential concentrates at the narrow band nearest the borehole. For example, for Rc≈100 meters, and rb≈0.1 meter, more than one-third of the pressure difference is dropped across the last one meter to the borehole core. Over one-half is dropped across a zone of radius ≈3 meters. The situation is even more pronounced for boreholes with smaller radii rb.
Mud filtrate and small coal particles can form a filter cake that will reduce or completely shut-down an exhaust well bore. The borehole output for the same face pressure can be considerably reduced by critical-zone pore-clogging, or colmatation. For example, it is estimated a tenfold decrease in penetrability in an area of radius 0.5 meter for rb≈0.1 meter results in a threefold decrease in the output. If the same decrease in penetrability takes place in an only slightly larger 0.2 meter radius zone, then the output is reduced by much less than before, e.g., 40%. Therefore, a principal benefit of acoustically vibrating the inside faces of the boreholes in porous and fractured media is to increase its permeability.